Re: [PATCH,RFC] RCU-based detection of stalled CPUs for Classic RCU (LTTng support)

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Thu, Oct 02, 2008 at 11:10:46AM -0400, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > On Thu, Oct 02, 2008 at 10:07:26AM +0200, Ingo Molnar wrote:
> > > > 
> > > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > > > 
> > > > > Hello!
> > > > > 
> > > > > This patch adds stalled-CPU detection to Classic RCU.  This capability 
> > > > > is enabled by a new config variable CONFIG_RCU_CPU_STALL_DETECTOR, 
> > > > > which defaults disabled.  This is a debugging feature, not something 
> > > > > that non-kernel-hackers would be expected to care about.  This feature 
> > > > > can detect looping CPUs in !PREEMPT builds and looping CPUs with 
> > > > > preemption disabled in PREEMPT builds.  This is essentially a port of 
> > > > > this functionality from the treercu patch.
> > > > > 
> > > > > One current shortcoming: on some systems, stalls are detected during 
> > > > > early boot, when we normally would not care about them.  My thought is 
> > > > > to add a call from late initialization to suppress stall detection 
> > > > > until the system is well along its way to being booted, but thought I 
> > > > > should check to see if there might already be something for this 
> > > > > purpose.
> > > > 
> > > > could you be a bit more specific, why do those warnings show up and why 
> > > > dont we care about them? There are things like networking that 
> > > > occasionally do an rcu_sync() and a stall could mean a bootup hang.
> > > 
> > > Hmmm...
> > > 
> > > Good point, I was just falling back on my old "we don't care about RCU
> > > stalls in boot-time code" rule from long ago.  It is entirely possible
> > > (in fact reasonably likely) that this rule no longer applies to Linux as
> > > it exists today.  So please see below for the console output.  Not all
> > > systems report this stall.  I have recently been running only on Power
> > > systems, will fire off on some x86s.  My kneejerk reaction was that the
> > > "stall" was really due to the clock-setting operation -- RCU stalls are
> > > based on get_seconds().
> > > 
> > > So maybe I need to change the stall-detection code to use jiffies
> > > instead.
> > > 
> > > Thoughts?
> > > 
> > 
> > You could possibly use get monotonic time, which should hopefully be a
> > bit better than its NTP-corrected alternatives.
> 
> OK.  I thought jiffies were monotonic (though not perfectly correlated
> to the passage of time).  This is a warning timeout, so does not need
> better than a few percent accuracy, as long as time never jumps too far.
> 

Well, jiffies should also be monotonic, but I don't know their exact
status wrt dyntick systems.

> But what API did you have in mind?
> 

Something like do_posix_clock_monotonic_gettime() in linux/time.h. I
think it should be ok after early boot.

> > Also, do you have all the tools you need to poinpoint the precise source
> > of RCU stalls ? I haven't looked at RCU trace, but I should say adding a
> > new tracepoint to instrument RCU callback execution at boot time and to
> > trace it with LTTng is close to trivial. (actually, very early kernel
> > boot is not so trivial as we would have to create a small kernel module
> > to activate lttng tracing, but tracing the early phases of userspace
> > bootup is trivial given we can use the userspace API).
> 
> I just dump stack, which historically worked pretty well.
> 
> Could you please tell me more about what you think should be added?
> 

I just created two patches which instrument RCU callbacks and call_rcu
in classic and preempt RCU to the -lttng tree to show how simple it is
once the LTTng infrastructure is in place.

Basically, all you need to do is to pull this tree (branch
2.6.27-rc8-lttng-0.31) :

git://git.kernel.org/pub/scm/linux/kernel/git/compudj/linux-2.6-lttng.git

And follow :
http://ltt.polymtl.ca/svn/trunk/lttv/QUICKSTART

Which will basically tell you to enable markers/tracepoints and leave
most ltt-related options to default (all in general setup menu).

wget http://ltt.polymtl.ca/lttng/ltt-control-0.53-02102008.tar.gz
extract, ./configure, make, make install

wget http://ltt.polymtl.ca/packages/lttv-0.10.0-pre15-12082008.tar.gz
extract, ./configure, make, make install
(see QUICKSTART for package dependencies. Will be automatically checked
by configure)

You may have to add a mount point for debugfs and load some kernel
modules (as explained in the QUICKSTART). Don't forget rcu-trace.ko.

To take a trace, once you are booted in the kernel :

ltt-armall
(check that the rcu_* markers are enabled by looking at the "state"
field of cat /proc/ltt | grep rcu)
lttctl -n trace -d -l /mnt/debugfs/ltt -t /tmp/trace
[Reproduce behavior to trace]
lttctl -n trace -R
# Produce a text dump
lttv -m textDump -t /tmp/trace

# Produce a text dump filtering only rcu events :
lttv -m textDump -t /tmp/trace \
  -e "event.name=rcu_classic_callback|event.name=rcu_classic_call_rcu|\
event.name=rcu_classic_call_rcu_bh|event.name=rcu_preempt_callback|\
event.name=rcu_preempt_call_rcu_sched|event.name=rcu_preempt_call_rcu"

Then some scripting could be required to turn the function pointers into
symbols. Keep you kernel vmlinux around to perform symbol resolution.
A quick trick is to use objdump -S --start-address=0x.... vmlinux|less
but I am certain there are much much better ways to do it. ;)

There is also the gui you can use to dig into the trace :
lttv-gui -t /tmp/trace

That should help you pinpoint which are the functions that took so much
time and which is their associated call_rcu caller IP.

If you run a trace with preempt rcu, the textdump should give you an
output like this :

[...]
rcu_preempt_call_rcu: 130.773191675 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 130.773194003 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 130.773195929 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 130.773197399 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 130.773200429 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802c6670, ip = 0xFFFFFFFF8023A45B }
rcu_preempt_call_rcu: 130.773213907 (/tmp/trace/cpu_5), 3961, 3961, bash, , 3960, 0x0, SYSCALL { func = 0xffffffff8024a980, ip = 0xFFFFFFFF8023937D }
rcu_preempt_call_rcu: 130.773215527 (/tmp/trace/cpu_5), 3961, 3961, bash, , 3960, 0x0, SYSCALL { func = 0xffffffff80238550, ip = 0xFFFFFFFF80239985 }
rcu_preempt_callback: 130.789025735 (/tmp/trace/cpu_2), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.789026731 (/tmp/trace/cpu_2), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.789050610 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.789051096 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802c6670 }
rcu_preempt_callback: 130.789052083 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff8024a980 }
rcu_preempt_callback: 130.789052713 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff80238550 }
rcu_preempt_callback: 130.805051097 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.805052318 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.805052822 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.805053434 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.805054226 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802c6670 }
rcu_preempt_callback: 130.805055339 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff8024a980 }
rcu_preempt_callback: 130.805056014 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff80238550 }
rcu_preempt_call_rcu: 131.442990122 (/tmp/trace/cpu_7), 3685, 3685, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 131.443615323 (/tmp/trace/cpu_6), 3687, 3687, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 131.446861604 (/tmp/trace/cpu_7), 3683, 3683, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 131.447487425 (/tmp/trace/cpu_5), 3681, 3681, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_callback: 131.473052317 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 131.473078626 (/tmp/trace/cpu_6), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 131.477001607 (/tmp/trace/cpu_7), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 131.477002435 (/tmp/trace/cpu_7), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_call_rcu: 133.443017217 (/tmp/trace/cpu_5), 3681, 3681, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 133.443636900 (/tmp/trace/cpu_6), 3687, 3687, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 133.446887423 (/tmp/trace/cpu_7), 3683, 3683, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 133.447509515 (/tmp/trace/cpu_7), 3685, 3685, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_callback: 133.473044696 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 133.473070649 (/tmp/trace/cpu_6), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 133.476994292 (/tmp/trace/cpu_7), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 133.476994961 (/tmp/trace/cpu_7), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_call_rcu: 134.085511140 (/tmp/trace/cpu_5), 3961, 3961, bash, , 3960, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 134.085545237 (/tmp/trace/cpu_5), 3981, 3981, bash, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 134.086124869 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802B8E2D }
rcu_preempt_call_rcu: 134.086135078 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802B8E2D }
rcu_preempt_call_rcu: 134.086143519 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802B8E2D }
rcu_preempt_call_rcu: 134.086206323 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802B8E2D }
rcu_preempt_call_rcu: 134.086526166 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
[...]

Basically, once you have the LTTng infrastructure in place, all that
needs to be added to RCU to do that is this (which is already in the
-lttng tree), plus a small LTTng "probe" module. If you want, you could
add a tracepoint in your stalled CPU detector and send the data through
LTTng so you have a starting point to dig from in large traces. From
that point, finding the root cause of long RCU-based CPU stalls becomes
trivial.


RCU : tracepoint instrumentation

Instrument RCU classic and RCU preempt callback execution and call_rcu.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
---
 include/trace/rcu.h |   31 +++++++++++++++++++++++++++++++
 kernel/rcuclassic.c |    4 ++++
 kernel/rcupreempt.c |    4 ++++
 3 files changed, 39 insertions(+)

Index: linux-2.6-lttng/include/trace/rcu.h
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-lttng/include/trace/rcu.h	2008-10-02 14:18:05.000000000 -0400
@@ -0,0 +1,31 @@
+#ifndef _TRACE_RCU_H
+#define _TRACE_RCU_H
+
+#include <linux/tracepoint.h>
+#include <linux/rcupdate.h>
+
+DEFINE_TRACE(rcu_classic_callback,
+	TPPROTO(struct rcu_head *head),
+	TPARGS(head));
+
+DEFINE_TRACE(rcu_classic_call_rcu,
+	TPPROTO(struct rcu_head *head, unsigned long ip),
+	TPARGS(head, ip));
+
+DEFINE_TRACE(rcu_classic_call_rcu_bh,
+	TPPROTO(struct rcu_head *head, unsigned long ip),
+	TPARGS(head, ip));
+
+DEFINE_TRACE(rcu_preempt_callback,
+	TPPROTO(struct rcu_head *head),
+	TPARGS(head));
+
+DEFINE_TRACE(rcu_preempt_call_rcu,
+	TPPROTO(struct rcu_head *head, unsigned long ip),
+	TPARGS(head, ip));
+
+DEFINE_TRACE(rcu_preempt_call_rcu_sched,
+	TPPROTO(struct rcu_head *head, unsigned long ip),
+	TPARGS(head, ip));
+
+#endif
Index: linux-2.6-lttng/kernel/rcuclassic.c
===================================================================
--- linux-2.6-lttng.orig/kernel/rcuclassic.c	2008-10-02 13:44:35.000000000 -0400
+++ linux-2.6-lttng/kernel/rcuclassic.c	2008-10-02 14:16:45.000000000 -0400
@@ -47,6 +47,7 @@
 #include <linux/notifier.h>
 #include <linux/cpu.h>
 #include <linux/mutex.h>
+#include <trace/rcu.h>
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 static struct lock_class_key rcu_lock_key;
@@ -138,6 +139,7 @@ void call_rcu(struct rcu_head *head,
 	head->func = func;
 	head->next = NULL;
 	local_irq_save(flags);
+	trace_rcu_classic_call_rcu(head, _RET_IP_);
 	rdp = &__get_cpu_var(rcu_data);
 	*rdp->nxttail = head;
 	rdp->nxttail = &head->next;
@@ -174,6 +176,7 @@ void call_rcu_bh(struct rcu_head *head,
 	head->func = func;
 	head->next = NULL;
 	local_irq_save(flags);
+	trace_rcu_classic_call_rcu_bh(head, _RET_IP_);
 	rdp = &__get_cpu_var(rcu_bh_data);
 	*rdp->nxttail = head;
 	rdp->nxttail = &head->next;
@@ -232,6 +235,7 @@ static void rcu_do_batch(struct rcu_data
 	while (list) {
 		next = list->next;
 		prefetch(next);
+		trace_rcu_classic_callback(list);
 		list->func(list);
 		list = next;
 		if (++count >= rdp->blimit)
Index: linux-2.6-lttng/kernel/rcupreempt.c
===================================================================
--- linux-2.6-lttng.orig/kernel/rcupreempt.c	2008-10-02 13:53:41.000000000 -0400
+++ linux-2.6-lttng/kernel/rcupreempt.c	2008-10-02 14:15:47.000000000 -0400
@@ -57,6 +57,7 @@
 #include <linux/byteorder/swabb.h>
 #include <linux/cpumask.h>
 #include <linux/rcupreempt_trace.h>
+#include <trace/rcu.h>
 
 /*
  * Macro that prevents the compiler from reordering accesses, but does
@@ -1108,6 +1109,7 @@ static void rcu_process_callbacks(struct
 	spin_unlock_irqrestore(&rdp->lock, flags);
 	while (list) {
 		next = list->next;
+		trace_rcu_preempt_callback(list);
 		list->func(list);
 		list = next;
 		RCU_TRACE_ME(rcupreempt_trace_invoke);
@@ -1122,6 +1124,7 @@ void call_rcu(struct rcu_head *head, voi
 	head->func = func;
 	head->next = NULL;
 	local_irq_save(flags);
+	trace_rcu_preempt_call_rcu(head, _RET_IP_);
 	rdp = RCU_DATA_ME();
 	spin_lock(&rdp->lock);
 	__rcu_advance_callbacks(rdp);
@@ -1141,6 +1144,7 @@ void call_rcu_sched(struct rcu_head *hea
 	head->func = func;
 	head->next = NULL;
 	local_irq_save(flags);
+	trace_rcu_preempt_call_rcu_sched(head, _RET_IP_);
 	rdp = RCU_DATA_ME();
 	spin_lock(&rdp->lock);
 	*rdp->nextschedtail = head;

The LTTng probe module looks like this :


LTTng trace RCU probe

RCU callback/call_rcu LTTng probe.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
---
 ltt/probes/Makefile    |    2 -
 ltt/probes/rcu-trace.c |   54 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 1 deletion(-)

Index: linux-2.6-lttng/ltt/probes/rcu-trace.c
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-lttng/ltt/probes/rcu-trace.c	2008-10-02 14:23:17.000000000 -0400
@@ -0,0 +1,54 @@
+/*
+ * ltt/probes/rcu-trace.c
+ *
+ * RCU tracepoint probes.
+ */
+
+#include <linux/module.h>
+#include <trace/rcu.h>
+
+#ifdef CONFIG_CLASSIC_RCU
+void probe_rcu_classic_callback(struct rcu_head *head)
+{
+	trace_mark_tp(rcu_classic_callback, rcu_classic_callback,
+		probe_rcu_classic_callback, "func %p", head->func);
+}
+
+void probe_rcu_classic_call_rcu(struct rcu_head *head, unsigned long ip)
+{
+	trace_mark_tp(rcu_classic_call_rcu, rcu_classic_call_rcu,
+		probe_rcu_classic_call_rcu, "func %p ip 0x%lX", head->func, ip);
+}
+
+void probe_rcu_classic_call_rcu_bh(struct rcu_head *head, unsigned long ip)
+{
+	trace_mark_tp(rcu_classic_call_rcu_bh, rcu_classic_call_rcu_bh,
+		probe_rcu_classic_call_rcu_bh, "func %p ip 0x%lX",
+		head->func, ip);
+}
+#endif
+
+#ifdef CONFIG_PREEMPT_RCU
+void probe_rcu_preempt_callback(struct rcu_head *head)
+{
+	trace_mark_tp(rcu_preempt_callback, rcu_preempt_callback,
+		probe_rcu_preempt_callback, "func %p", head->func);
+}
+
+void probe_rcu_preempt_call_rcu(struct rcu_head *head, unsigned long ip)
+{
+	trace_mark_tp(rcu_preempt_call_rcu, rcu_preempt_call_rcu,
+		probe_rcu_preempt_call_rcu, "func %p ip 0x%lX", head->func, ip);
+}
+
+void probe_rcu_preempt_call_rcu_sched(struct rcu_head *head, unsigned long ip)
+{
+	trace_mark_tp(rcu_preempt_call_rcu_sched, rcu_preempt_call_rcu_sched,
+		probe_rcu_preempt_call_rcu_sched, "func %p ip 0x%lX",
+		head->func, ip);
+}
+#endif
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Mathieu Desnoyers");
+MODULE_DESCRIPTION("RCU Tracepoint Probes");
Index: linux-2.6-lttng/ltt/probes/Makefile
===================================================================
--- linux-2.6-lttng.orig/ltt/probes/Makefile	2008-10-02 14:11:20.000000000 -0400
+++ linux-2.6-lttng/ltt/probes/Makefile	2008-10-02 14:18:16.000000000 -0400
@@ -9,7 +9,7 @@ CFLAGS_REMOVE_lockdep-trace.o = -pg
 endif
 
 obj-$(CONFIG_LTT_TRACEPROBES)	+= kernel-trace.o mm-trace.o fs-trace.o \
-				ipc-trace.o lockdep-trace.o
+				ipc-trace.o lockdep-trace.o rcu-trace.o
 ifeq ($(CONFIG_NET),y)
 CFLAGS_REMOVE_net-trace.o = -pg
 obj-$(CONFIG_TRACEPROBES)	+= net-trace.o


Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68